There is an ever-increasing demand for improving light management and luminous efficiency in many illumination applications. This hypothesis is especially applicable to LED (Light-Emitting Diode) illumination. It is necessary to utilize efficient optics with point-like light sources such as LEDs in order to achieve the required lighting performance. Solutions exploiting diffractive optics or other sophisticated micro-optic structures provide a range of new possibilities for advanced illumination including dimensional benefits and cost-effective integrations.
Generally, the fabrication of diffractive (DO) or other micro-optic structures is completed by means of lithographic processes such as mask lithography, direct laser beam, or e-beam writing. These methods have certain disadvantages, which critically limit the fabrication of preferred structures. The most critical issues relate to fabrication of complex three-dimensional shapes (like blazed gratings), flexible orientations/modulation capability, as well as large surface patterning. Lithographic methods do not perform well with blazed/slanted or hybrid profiles. On the other hand, wafers are generally provided in very limited sizes (e.g. 6 or 8 inch sizes), which sets additional burden for manufacturing of large surfaces with microstructures.
Conventional lithographic methods also primarily utilize resist layers, which requires adoption of several process steps such as etching, developing, etc. Thus the total process speed is limited and optical measurement can't be done simultaneously. In imprinting, the patterning is typically completed on the soft resist layers, by means of thermal or UV (Ultraviolet) curing. Consequently, the vertical positioning cannot be controlled in a clever manner, and a longer process time is thus required for the depth control. Most thermoplastic resists are softer requiring etching.
Recently, micro-machining has become more precise, allowing micro-optic structures to be cut with high quality diamond tools. However, there are still critical limitations, such as flexibility of structural orientations/modulation, difficulties in fabricating larger structures, large machining tolerances, etc. relating to the feasibility of such solutions.
Considering the replication of various microstructures in general and after manufacturing a master mold containing those, the used methods may include UV or thermal casting, hot embossing and injection molding, for example.
In UV and thermal casting a UV or thermally curable polymer resin is spread onto a base material, for example, PMMA. The master, e.g. a PDMS (polydimethylsiloxane) silicone stamper, is then placed in contact with the adhesive and immediately cured with an UV-lamp, or in an oven, respectively. After curing, the shim and the replica can be separated.
Hot embossing lithography, e.g. flatbed and roll-to-roll embossing, is an imprinting method wherein microstructures are formed on a substrate also using a master mold. The replication material can either be in the form of a sheet of plastic foil or a thin film, spin-coated onto a substrate. During embossing, the master, e.g. a Ni-shim, is placed in contact with the plastic film, and pressure is applied on the film being heated above its glass transition temperature Tg. After removal of the pressure, the film is cooled down and the Ni shim is removed, leaving a high quality replica of the micro-structure behind.
Injection molding is an established technology for high-speed mass production of plastic components, e.g. compact discs. A master such as a Ni-shim containing the microstructure to be replicated is mounted on one side of the mold, and preheated plastic is injected into the mold. A pressure of several MPa is then applied. After rapid cooling, the molded part is extracted.
Also screenprinting (silkscreening) techniques wherein white reflective spots are printed on a carrier surface have been presented for implementing microstructures. The generated spots generally provide rather limited optical performance.
Despite of various existing methods for manufacturing different microstructures on a carrier surface the aforesaid problems have remain at least partially unsolved in the context of high-precision micro-optic products.